docs: def _get_derivative(self, C, sa1180, vs30):
          """
          Returns equation 30 page 1047
          """
          derAmp = np.zeros_like(vs30)
          n = self.CONSTS['n']
          c = C['c']
          b = C['b']
          idx = vs30 < C['vlin']
          derAmp[idx] = (b * sa1180[idx] * (-1./(sa1180[idx]+c) +
                         1./(sa1180[idx] + c*(vs30[idx]/C['vlin'])**n)))
          return derAmp